The Social Security Administration (SSA) provides income support and medical benefits for adults with mental retardation unable to perform substantial gainful activity through the Disability Insurance (DI) program and the Supplemental Security Income (SSI) program. SSI benefits are also provided to families of children and adolescents who evidence marked and severe restrictions in functioning because of mental retardation. This report is focused on specifying criteria for the determination of mental retardation for SSI/DI eligibility. It examines the contextual issues affecting SSA disability benefit programs, with the recognition that any evaluation of the current determination process for mental retardation is likely to have public policy effects. These effects are discussed in Chapter 2. Chapter 3 examines whether current IQ tests adequately reflect accepted concepts of intelligence. Chapter 4 explores how adaptive functioning is best defined and assessed. Chapter 5 investigates the relationships between measures of intelligence and adaptive behaviors. Chapter 6, on differential diagnosis, explains how the conditions that share signs and symptoms with mental retardation are best distinguished from it. Recommendations related to intelligence assessment, assessment of adaptive behavior, for combining IQ and adaptive behavior, differential diagnosis, for removing disincentives to employment, and for future research are provided. (Contains more than 500 references.) (CR)